High selectivity nitride removal process based on selective polymer deposition

ABSTRACT

A silicon nitride cap on a gate stack is removed by etching with a fluorohydrocarbon-containing plasma subsequent to formation of source/drain regions without causing unacceptable damage to the gate stack or source/drain regions. A fluorohydrocarbon-containing polymer protection layer is selectively deposited on the regions that are not to be etched during the removal of the nitride cap. The ability to remove the silicon nitride material using gas chemistry, causing formation of a volatile etch product and protection layer, enables reduction of the ion energy to the etching threshold.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.14/615,067 filed Feb. 5, 2015, the complete disclosure of which isexpressly incorporated herein by reference in its entirety for allpurposes.

FIELD

The present disclosure relates to the physical sciences, and, moreparticularly, to the fabrication of semiconductor devices.

BACKGROUND

The fabrication of field effect transistors (FETs) can involve theformation of a silicon nitride (Si₃N₄) cap atop a gate stack includinggate electrode and gate dielectric layers. The silicon nitride cap isadded to the top of the gate stack during the stack deposition processand must remain sufficiently intact after patterning the same so as toimpede epitaxial growth of semiconductor materials at the top of thegate stack during formation of source/drain regions. Silicon germanium(SiGe) and carbon doped silicon (Si:C) are among the materials employedfor forming source/drain regions of silicon-based pFET and nFET devices,respectively. Subsequent to formation of source/drain regions, thenitride cap must be fully removed from the gate stack without damagingexposed portions of the source/drain regions. Wafer wide removal can beattempted using conventional chemistries, for example CH₃F/O₂(fluorohydrocarbon) and low ion energy platforms such as the TEL (TokyoElectron Limited) RLSA (radial line slot antenna) and Lam Research KIYO®conductor etch system. A fluorohydrocarbon plasma employed foranisotropic etching of silicon nitride is selective to silicon oxide.Selectivity to silicon, while not inherent, is based on the formation ofsilicon oxide on silicon, thereby preventing further erosion of thesilicon. High selectivity to silicon at the nanoscale level is notobtained using such technology. The removal of the nitride cap whileavoiding damage to the gate and/or the source/drain regions in anefficient and effective manner is a goal of those in the semiconductorprocessing industry.

Some types of field effect transistors (FETs) have three-dimensional,non-planar configurations including fin-like structures extending abovesubstrates. Such field effect transistors are referred to as FinFETs.The substrates may include semiconductor on insulator (SOI) substratesor bulk semiconductor substrates. Silicon fins are formed in someFinFETs on substrates via known technology such as sidewall imagetransfer (SIT). FinFET structures including SOI substrates can beformed, in part, by selectively etching the crystalline silicon layersdown to the oxide or other insulating layers thereof followingphotolithography. Active fin heights are set by SOI thickness whenemploying SOI substrates. In bulk FinFETs, active fin height is set byoxide thickness and etched fin height. The gates of FinFETs can beformed using a “gate-first” process wherein a gate stack and spacers areformed prior to selective epitaxial growth wherein source and drainregions are enlarged.

SUMMARY

Principles of the present disclosure provide an exemplary fabricationmethod that includes obtaining a FET structure comprising asemiconductor substrate, a gate stack on the substrate, source/drainregions operatively associated with the gate stack, and a siliconnitride cap of the gate stack. The method further includes generating afluorohydrocarbon-containing plasma selective to silicon bydecomposition of C_(x)H_(y)F_(z) wherein x is an integer selected from3, 4, 5 and 6, y and z are positive integers, and y is greater than z.The silicon nitride cap is etched anisotropically by employing thefluorohydrocarbon-containing plasma to form a first hydrofluorocarbonpolymer layer having a first thickness on the source/drain regions and asecond hydrofluorocarbon polymer layer having a second thickness on thesilicon nitride cap, the first thickness being greater than the secondthickness, the second hydrofluorocarbon polymer layer further comprisinga volatile nitrogen-containing compound formed by interaction of thefluorohydrocarbon-containing plasma with the silicon nitride comprisingthe silicon nitride cap.

An exemplary structure includes a semiconductor substrate, a gate stackon the substrate, a channel region beneath the gate stack, spacersadjoining the gate stack, and source/drain regions operativelyassociated with the gate stack and channel region, the source/drainregions having top surfaces. A fluorohydrocarbon-containing polymerlayer directly contacts the top surfaces of the source/drain regions andcovers the source/drain regions.

As used herein, “facilitating” an action includes performing the action,making the action easier, helping to carry the action out, or causingthe action to be performed. Thus, by way of example and not limitation,instructions executing on one processor might facilitate an actioncarried out by instructions executing on a remote processor, by sendingappropriate data or commands to cause or aid the action to be performed.For the avoidance of doubt, where an actor facilitates an action byother than performing the action, the action is nevertheless performedby some entity or combination of entities.

Fabrication methods as disclosed herein can provide substantialbeneficial technical effects. For example, one or more embodiments mayprovide one or more of the following advantages:

-   -   High selectivity nitride gate cap removal;    -   Avoidance of damage to source/drain regions and gate stack;    -   Reduction of required ion energy to the etching threshold by        chemical removal of the nitride gate cap;    -   High macro-to-macro and wafer-wide uniformity ensuring        quasi-ideal device characteristics;    -   Very low CD (critical dimension) bias (litho->final);    -   Lower substrate temperature processing range.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a FET structure including a gatestack and operatively associated source/drain regions;

FIG. 2A is a schematic illustration of the structure of FIG. 1 followinganisotropic etch;

FIG. 2B is a schematic illustration of the structure of FIG. 2Afollowing cleaning of the etched structure;

FIG. 3 is a schematic illustration including top and sectional views ofa FET structure including a soft photoresist layer thereon;

FIG. 4A is a schematic illustration the structure of FIG. 3 showing thestructure of FIG. 3 following a conventional, single-carbonfluorohydrocarbon etch;

FIG. 4B is a schematic illustration of the structure of FIG. 3 followinguse of an anisotropic plasma etch that causes the selective depositionof a polymer layer and subsequent removal of the polymer layer, and

FIG. 5 is a flow chart showing an exemplary sequence of steps forfabricating a FET structure.

DETAILED DESCRIPTION

An anisotropic silicon nitride etch provides selectivity to silicon byforming a fluorohydrocarbon-containing polymer on silicon surfaces.Selective fluorohydrocarbon deposition is employed to provideselectivity to non-nitride surfaces. The fluorohydrocarbon-containingpolymer interacts with silicon nitride to form a volatile compound,thereby enabling etching of silicon nitride. Thefluorohydrocarbon-containing polymer does not interact with silicon, andprotects silicon-based source/drain regions from the plasma. Theanisotropic silicon nitride etch can be employed to etch silicon nitrideselective to silicon and silicon oxide in any dimension, including smalldimensions less than fifty nm. The processes discussed below areapplicable to fabrication of nFET and pFET structures. FIGS. 1-2illustrate exemplary structures that may be obtained sequentially infabricating a structure, it being appreciated that additional steps areperformed to obtain the structure shown in FIG. 1 and that further stepsare performed in some embodiments once the structure shown in FIG. 2 hasbeen obtained. The drawings are not necessarily to scale.

FIG. 1 shows a FET structure 10 including a semiconductor substrate 10,which in some embodiments is a monocrystalline silicon layer formed onan electrically insulating layer (not shown) source/drain regions 14formed within the substrate 10, and a gate stack operatively associatedwith the source/drain regions 14. The region of the substrate 10 beneaththe gate stack forms a channel region. The gate stack is adjoined byspacers 16. The spacers 16 comprise multiple layers of electricallyinsulating materials in some embodiments. A cap 18 adjoins the topsurface of the gate stack. The cap on the gate stack includes siliconnitride. The silicon nitride can be stoichiometric having an atomicratio of 3:4 between silicon and nitrogen, or can be non-stoichiometric.

Gate stacks can be fabricated, for example, by forming a stack includinga gate dielectric layer 22 and a gate conductor layer 20 on a portion ofthe substrate suitable for use as a channel. The sidewalls of the gatedielectric 22, the gate electrode or conductor 20, and the gate cap 18can be vertically coincident, i.e., coincide among one another in a topdown view, i.e., a view from above in a direction perpendicular to thehorizontal plane between the semiconductor material portion 12 and thegate dielectric layer 22. In the exemplary embodiment shown in FIG. 1,the cap 18 extends within the area defined by the spacers 16 and abovethe top surface of the spacers 16.

Gate stacks including the gate dielectric layer 22 and gate conductorlayer 20 are formed in one or more embodiments using a gate firstintegration scheme as known in the art. In such embodiments, the gatestack and nitride cap are formed prior to formation of the source/drainregions, which are then deposited epitaxially in some embodiments.Chemical vapor deposition (CVD) is one technique for epitaxiallydepositing layers of SiGe and Si:C. The silicon nitride cap 18 impedesepitaxial growth of such materials during epitaxial formation of thesource/drain regions, which may be either n-type or p-type, the oppositeconductivity type of the associated channel beneath the gate dielectriclayer 22. The source/drain regions may comprise other materials, such asGroup III-V semiconductor materials, in some embodiments. The structure10 as schematically illustrated in FIG. 1 is obtained in someembodiments. It will be appreciated that other transistor structures mayalternatively be obtained. For example, the source/drain regions 14 areraised in some embodiments with respect to the dielectric layer 22.

Referring to FIG. 1, the exemplary structure 10 is placed into a processchamber configured for a plasma etch, i.e., a reactive ion etch. Ananisotropic etch employing a fluorohydrocarbon-containing plasma isperformed on the exemplary structure. The composition of the gassupplied into the process chamber includes one or more compositions ofC_(x)H_(y)F_(z), wherein x is an integer selected from 3, 4, 5, and 6, yand z are positive integers, and y is greater than z. For example, thefluorohydrocarbon gas employed in the present disclosure can include oneor more of C₃H₅F₃, C₃H₆F₂, C₃H₇F, C₃H₄F₂, C₃H₅F, C₃H₃F, C₄H₆F₄, C₄H₇F₃,C₄H₈F₂, C₄H₉F, C₄H₅F₃, C₄H₆F₂, C₄H₇F, C₄H₄F₂, C₄H₅F, C₅H₇F₅, C₅H₈F₄,C₅H₉F₃, C₅H₁₀F₂, C₅H₁₁F, C₅H₆F₄, C₅H₇F₃, C₅H₈F₂, C₅H₉F, C₅H₅F₃, C₅H₆F₂,C₅H₇F, C₆H₈F₆, C₆H₉F₅, C₆H₁₀F₄, C₆H₁₁F₃, C₆H₁₂F₂, C₆H₁₃F, C₆H₇F₅,C₆H₈F₄, C₆H₉F₃, C₆H₁₀F₂, C₆H₁₁F, C₆H₆F₄, C₆H₇F₃, C₆H₈F₂, and C₆H₉F.Correspondingly, the fluorohydrocarbon-containing plasma includes ionsof C_(x)H_(y)F_(z). Optionally, the composition of the gas supplied intothe process chamber can further include O₂, CO, CO₂, N₂, Ar, H₂, He orcombinations thereof. In other words, the fluorohydrocarbon-containingplasma optionally includes a plasma of O₂, CO, CO₂, N₂, Ar, H₂, He orcombinations thereof in addition to the plasma of C_(x)H_(y)F_(z). US2013/0105916, which is incorporated by reference herein, describes theseand other fluorohydrocarbon gases that may be employed to provideselectivity to non-nitride surfaces.

Non-limiting specific examples of C_(x)H_(y)F_(z), wherein x is aninteger selected from 3, 4, 5, and 6, y and z are positive integers, andy is greater than z, include alkanes, alkenes, and alkynes.

In one embodiment, the fluorohydrocarbon gas can include one or morealkane fluorohydrocarbon gas having the formula of C_(x)H_(y)F_(z),wherein x is an integer selected from 3, 4, and 5, y and z are positiveintegers, and y is greater than z. The one or more alkanefluorohydrocarbon gas can include, but are not limited to: saturatedliner fluorohydrocarbons shown by C₃H₇F such as 1-fluoropropane,2-fluoropropane; saturated liner fluorohydrocarbons shown by C₃H₆F₂ suchas 1,1-difluoropropane, 2,2-difluoropropane, 1,2-difluoropropane,1,3-difluoropropane; saturated liner fluorohydrocarbons shown by C₃H₅F₃such as 1,1,1-trifluoropropane, 1,1,2-trifluoropropane,1,1,3-trifluoropropane, 1,2,2-trifluoropropane; saturated cyclicfluorohydrocarbon shown by C₃H₅F such as fluorocyclopropane; saturatedcyclic fluorohydrocarbon shown by C₃H₄F₂ such as1,2-difluorocycloproapne; saturated liner fluorohydrocarbons shown byC₄H₉F such as 1-fluorobutane, 2-fluorobutane; saturated linerfluorohydrocarbons shown by C₄H₈F₂ such as 1-fluoro-2-methylpropane,1,1-difluorobutane, 2,2-difluorobutane, 1,2-difluorobutane,1,3-difluorobutane, 1,4-difluorobutane, 2,3-difluorobutane,1,1-difluoro-2-methylpropane, 1,2-difluoro-2-methylpropane,1,3-difluoro-2-methylpropane; saturated liner fluorohydrocarbons shownby C₄H₇F₃ such as 1,1,1-trifluorobutane,1,1,1-trifluoro-2-methylpropane, 1,1,2-trifluorobutane,1,1,3-trifluorobutane, 1,1,4-trifluorobutane, 2,2,3-trifluorobutane,2,2,4-trifluorobutane, 1,1,2-trifluoro-2-methylpropane; saturated linerfluorohydrocarbons shown by C₄H₆F₄ such as 1,1,1,2-tetrafluorobutane,1,1,1,3-tetrafluorobutane, 1,1,1,4-tetrafluorobutane,1,1,2,2-tetrafluorobutane, 1,1,2,3-tetrafluorobutane,1,1,2,4-tetrafluorobutane, 1,1,3,3-tetrafluroobutane,1,1,3,4-tetrafluorobutane, 1,1,4,4-tetrafluorobutane,2,2,3,3-tetrafluorobutane, 2,2,3,4-tetrafluorobutane,1,2,3,4-tetrafluorobutane, 1,1,1,2-tetrafluoro-2-methylpropane,1,1,1,3-tetrafluoro-2-methylpropane,1,1,2,3-tetrafluoro-2-methylpropane,1,1,3,3-tetrafluoro-2-methylpropane; saturated cyclic fluorohydrocarbonshown by C₄H₇F such as fluorocyclobutane; saturated cyclicfluorohydrocarbons shown by C₄H₆F₂ such as 1,1-difluorocyclobutane,1,2-difluorocyclobutane, 1,3-difluorocyclobutane; saturated cyclicfluorohydrocarbon shown by C₄H₅F₃ such as 1,1,2-trifluorocyclobutane,1,1,3-triflurocyclobutane; saturated liner fluorohydrocarbons shown byC₅H₁₁F such as 1-fluoropentane, 2-fluoropentane, 3-fluoropentane,1-fluoro-2-methylbutane, 1-fluoro-3-methylbutane,2-fluoro-3-methylbutane, 1-fluoro-2,2-dimethylpropane; saturated linerfluorohydrocarbons shown by C₅H₁₀F₂ such as 1,1-difluoropenatne,2,2-difluoropentane, 3,3-difluoropentane, 1,2-difluoropentane,1,3-difluoropentane, 1,4-difluoropentane, 1,5-difluoropentane,1,1-difluoro-2-methylbutane, 1,1-difluoro-3-methylbutane,1,2-difluoro-2-methylbutane, 1,2-difluoro-3-methylbutane,1,3-difluoro-2-methylbutane, 1,3-difluoro-3-methylbutane,1,4-difluoro-2-methylbutane, 2,2-difluoro-3-methylbutane,2,3-difluoro-2-methylbutane, 1,1-difluoro-2,2-dimethylpropane,1,3-difluoro-2,2-dimethylproapne, 1-fluoro-2-fluoromethylbutane;saturated liner fluorohydrocarbons shown by C5H9.3 such as1,1,1-trifluoropentane, 1,1,2-trifluoropentane, 1,1,3-trifluoropentane,1,1,4-trifluoropentane, 1,1,1-trifluoro-2-methylbutane,1,1,2-trifluoro2,3-dimethylpropane; saturated cyclic fluorohydrocarbonsshown by C₅H₉F such as fluorocyclopentane, 1-fluoro-2-methylcyclobutane,1-fluoro-3-methylcyclobutane, (fluoromethyl)-cyclobutane; saturatedcyclic fluorohydrocarbons shown by C₅H₈F₂ such as1,2-difluorocyclopentane, 1,3-difluorocyclopentane,1,1-difluoro-2-methylcyclobutane, 1,1-difluoro-3-methylcyclobutane;saturated cyclic fluorohydrocarbons shown by C₅H₇F₃ such as1,1,2-trifluorocyclopentane and 1,2,3,trifluorocyclopentane.

Additionally or alternatively, the fluorohydrocarbon gas can include oneor more alkene fluorohydrocarbon gas having the formula ofC_(x)H_(y)F_(z), wherein x is an integer selected from 3, 4, and 5, yand z are positive integers, and y is greater than z. The one or morealkene fluorohydrocarbon gas can include, but are not limited to:unsaturated liner fluorohydrocarbons shown by C₃H₅F such as3-fluoropropene, 1-fluoropropene, 2-fluoropropene; unsaturated linerfluorohydrocarbons shown by C₃H₄F₂ such as 1,1-difluoropropene,3,3-difluoropropene; unsaturated cyclic fluorohydrocarbons shown byC₃H₃F such as 3-fluorocyclopropene, 1-fluorocyclopropene; unsaturatedliner fluorohydrocarbons shown by C₄H₇F such as 1-fluorobutene,2-fluorobutene, 3-fluorobutene, 4-fluorobutene, 1-fluoro-2-butene,2-fluoro-2-butene, 1-fluoro-2-methylpropene, 3-fluoro-2-methylpropene,2-(fluoromethyl)-propene; unsaturated liner fluorohydrocarbons shown byC₄H₆F₂ such as 1,1-difluoro-2-methylpropene,3,3-difluoro-2-methylpropene, 2-(fluoromethyl)-fluoropropene,3,3-difluorobutene, 4,4-difluorobutene, 1,2-difluorobutene,1,1-difluoro-2-butene, 1,4-difluoro-2-butene; unsaturated linerfluorohydrocarbons shown by C₄H₅F₃ such as 4,4,4-trifluorobutene,3,3,4-trifluorobutene, 1,1,1-trifluoro-2-butene,1,1,4-trifluoro-2-butene; unsaturated cyclic fluorohydrocarbons shown byC₄H₅F such as 1-fluorocyclobutene, 3-fluorocyclobutene; unsaturatedcyclic fluorohydrocarbons shown by C₄H₄F₂ such as3,3-difluorocyclobutene, 3,4-difluorocyclobutene; unsaturated linerfluorohydrocarbons shown by C₅H₉F such as 1-fluoropentene,2-fluoropenten, 3-fluoropenten, 4-fluoropentene, 5-fluoropenten,1-fluoro-2-pentene, 2-fluoro-2-pentene, 3-fluoro-2-pentene,4-fluoro-2-pentene, 5-fluoro-2-pentene, 1-fluoro-2-methylbutene,1-fluoro-3-methylbutene, 3-fluoro-2-methylbutene,3-fluoro-3-methylbutene, 4-fluoro-2-methylbutene,4-fluoro-3-methylbutene, 1-fluoro-2-methyl-2-butene,1-fluoro-3-methyl-2-butene, 2-fluoro-3-methyl-2-butene,2-(fluoromethyl)-butene; unsaturated liner fluorohydrocarbons shown byC₅H₈F₂ such as 3,3-difluoropentene, 4,4-difluoropentene,5,5-difluoropentene, 1,2-difluoropentene, 3,4-difluoropentene,3,5-difluoropentene, 4,5-difluoropentene; unsaturated cyclicfluorohydrocarbons shown by C₅H₇F such as 1-fluorocyclopentene,3-fluorocylopentene, 4-fluorocyclopentene; unsaturated cyclicfluorohydrocarbons shown by C₅H₆F₂ such as 3,3-difluorocyclopentene,4,4-difluorocyclopentene, 1,3-difluorocyclopentene,1,4-difluorocyclopentene, 3,5-difluorocyclopentene.

Additionally or alternatively, the fluorohydrocarbon gas can include oneor more alkyne fluorohydrocarbon gas having the formula ofC_(x)H_(y)F_(z), wherein x is an integer selected from 3, 4, and 5, yand z are positive integers, and y is greater than z. The one or morealkyne fluorohydrocarbon gas can include, but are not limited to:unsaturated liner fluorohydrocarbon shown by C₃H₃F such as3-fluoropropyne; unsaturated liner fluorohydrocarbon shown by C₃H₂F₂such as 3,3-difluoropropyne; unsaturated liner fluorohydrocarbons shownby C₄H₅F such as 3-fluorobutyne, 4-fluorobutyne, 1-fluoro-2-butyne;unsaturated liner fluorohydrocarbons shown by C₄H₄F₂ such as3,3-difluorobutyne, 4,4-difluorobutyne, 3,4-difluorobutyne,1,4-difluoro-2-butyne; unsaturated liner fluorohydrocarbons shown byC₅H₇F such as 3-fluoropentyne, 4-fluoropentyne, 5-fluoropentyne,1-fluoro-2-pnetyne, 4-fluoro-2-pentyne, 5-fluoro-2-pentyne,3-(fluoromethyl)-butyne; unsaturated liner fluorohydrocarbons shown byC₅H₆F₂ such as 3,3-difluoropentyne, 4,4-difluoropentyne,5,5-difluoropentyne, 3,4-difluoropentyne, 4,5-difluoropentyne,1,1-difluoro-2-pentyne, 4,4-difluor-2-pentyne, 5,5-difluoro-2-pentyne,4,5-difluoro-2-pentyne, 3-(difluoromethyl)-butyne,3-(fluoromethyl)-4-fluorobutyne.

Upon reaction with silicon in the source/drain regions 14 and siliconnitride in the cap 18, the fluorohydrocarbon-containing plasma generatesa significant quantity of polymers on the top surfaces of thesource/drain regions 14. The quantity of polymers on the top surfaces ofthe source/drain regions is significant enough to be measurableemploying analytical instruments available in the art such as Augerelectron spectroscopy (AES) or x-ray photoelectron spectroscopy (XPS).The thicknesses of the polymers on the top surfaces of the source/drainregions 14 can be from 0.1 nm to 3 nm depending on the processconditions employed to generate the fluorohydrocarbon-containing plasma.

Specifically, a first fluorohydrocarbon-containing polymer layer 30, asshown in FIG. 2A, is formed on the top surface of the source/drainregions 14 and a second fluorohydrocarbon-containing polymer (not shown)is formed on the top surfaces of the silicon nitride cap 18. The firstfluorohydrocarbon-containing polymer and the secondhydrocarbon-containing polymer include carbon, hydrogen, and fluorine.Further, if O₂ or another oxygen-containing gas is supplied into theprocess chamber as one of the source gases, the firstfluorohydrocarbon-containing polymer and the secondhydrocarbon-containing polymer include oxygen.

In one embodiment, the first fluorohydrocarbon-containing polymer layer30 includes carbon at an atomic concentration between 30% and 40%,hydrogen at an atomic concentration between 40% and 50%, fluorine at anatomic concentration between 5.0% and 10.0%, and oxygen at an atomicconcentration less than 5%. The stoichiometry of the polymer layerformed on SiGe or Si:C does not differ from that of silicon, though thedeposited thickness of film may be thinner or thicker (based oncondition). Silicon germanium source/drain regions employed inaccordance with one or more embodiments comprise Si_(x)Ge_(1-x), where xis any value between 0 and 1. Ion Energy and process temperature arelower than those employed where the polymer layer is formed only onsilicon.

The second fluorohydrocarbon-containing polymer formed on the topsurface of the nitride cap 18 includes carbon, hydrogen, fluorine,optionally oxygen, and additionally includes nitrogen. Thus, the secondfluorohydrocarbon-containing polymer includes a nitrogen-containingcompound formed by interaction of the fluorohydrocarbon-containingplasma with the silicon nitride. The nitrogen-containing compound is avolatile compound including C, H, F, and N. As used herein, a volatilecompound refers to a compound that vaporizes in vacuum at 297.3° K.Thus, the second fluorohydrocarbon-containing polymer volatilizes and isremoved from the top surface of the silicon nitride cap 18 during theanisotropic etch.

The thickness of the first fluorohydrocarbon-containing polymer layer 30on the source/drain regions 14 during a steady state of the anisotropicetch is herein referred to as a first thickness t1. As used herein, asteady state of an etch refers to a state at which the thicknesses ofthe etch byproducts such as polymers do not change in time. FIG. 2Aschematically illustrates the structure 10 during the steady state ofthe anisotropic etch.

The bottom portion of the second fluorohydrocarbon-containing polymerinteracts with the silicon nitride material in the silicon nitride cap18 and subsequently volatilizes. Thus, the thickness t2 of the secondfluorohydrocarbon-containing polymer on the cap 18 remains insignificantand does not impede the interaction of the fluorohydrocarbon-containingplasma with the silicon nitride material in the silicon nitride cap 18.In contrast, the first fluorohydrocarbon-containing polymer does notinteract with the underlying material comprising the source/drainregions 14. Thus, the first fluorohydrocarbon-containing polymer layer30 impedes the interaction of the fluorohydrocarbon-containing plasmawith the silicon-containing source/drain regions 14. Because the firstfluorohydrocarbon-containing polymer does not interact with underlyingsilicon-containing material comprising the source/drain regions 14, thefirst thickness t1 is not less than the second thickness t2 of thepolymer that is formed over the cap 18.

Because the first thickness t1 is not less than the second thickness t2,the fluorohydrocarbon plasma anisotropically etches the silicon nitridecomprising the cap 18 as well as the spacers 16 at an etch rate that isgreater than corresponding etch rates for the source/drain regions 14.The combination of the differences between the thicknesses of thevarious fluorohydrocarbon-containing polymers and the reaction betweenthe second fluorohydrocarbon-containing polymer with the underlyingsilicon nitride material provides high selectivity to the anisotropicetch process so that the anisotropic etch removes silicon nitride withhigh selectivity to the silicon-containing materials comprising thesource/drain regions 14 and any other silicon-containing regions thatmay be exposed to the plasma during the etch process. The process isapplicable to source/drain regions comprising III-V materials or anyother semiconductor materials employable as source/drain regions thatcan benefit from the high selectivity obtained by using the disclosedprocess.

In one exemplary embodiment, the anisotropic etch can be employed toperform a silicon nitride etch process that is selective to silicongermanium (Si_(x)Ge_(1-x)) with a high selectivity. As used herein, theselectivity of the silicon nitride etch process relative to silicongermanium is the ratio of a second etch depth d2 to a first etch depthd1, the etch depths corresponding to the amounts of material removedfrom the different elements of the structure 10 shown in FIG. 1. In oneembodiment, the selectivity (d2/d1) of the silicon nitride etch processemploying the fluorohydrocarbon-containing plasma described aboverelative to silicon germanium can be greater than thirty (30).

In conventional silicon nitride etch processes, the number of carbonatoms in the plasma precursor gas is less than three. Further, thenumber of fluorine atoms in the ions of the conventional plasma isgreater than the number of hydrogen atoms in the molecules of aconventional plasma. The selectivity of the conventional silicon nitrideetch process relative to silicon is provided indirectly by includinghydrogen ions in the conventional plasma, which reduces the siliconetchant supply and converts the surface portion of the exposed siliconinto silicon oxide and prevents further etching of silicon.

In contrast, the number of carbon ions in the molecule of thefluorohydrocarbon-containing plasma of the present disclosure is atleast three (3). Further, the number of hydrogen atoms in the moleculeof the fluorohydrocarbon-containing plasma is greater than the number offluorine atoms in the molecule of the fluorohydrocarbon-containingplasma in the present disclosure. Thus, the atomic percentages of carbonand hydrogen in the first and second fluorohydrocarbon-containingpolymers increase over the corresponding atomic percentages in anypolymer of conventional silicon nitride etch processes. At the sametime, the atomic percentage of fluorine in the first and secondfluorohydrocarbon-containing polymers is less than the correspondingatomic percentage in any polymer of conventional silicon nitride etchprocesses. The increased carbon content and decreased fluorine contentrenders the first fluorohydrocarbon-containing polymer as depositednon-etchable by the fluorohydrocarbon-containing plasma givenappropriate plasma conditions. However, the secondfluorohydrocarbon-containing polymer is reduced by formation of anitrogen-containing volatile compound that is formed by interactionbetween the second fluorohydrocarbon-containing polymer and theunderlying silicon nitride material. Thus, the mechanism for providingselectivity in the silicon nitride etch relative to silicon germaniumor, in another embodiment silicon carbide, is deposition offluorohydrocarbon-containing polymer on silicon germanium or siliconcarbide surfaces that is not etchable by thefluorohydrocarbon-containing plasma.

In addition to the change in the quality of thefluorohydrocarbon-containing polymer of the present disclosure relativeto any polymer deposits generated in conventional silicon nitride etchprocesses, the amount of fluorohydrocarbon-containing polymers per unitof etching depth for a silicon nitride portion increases significantlyover the amount of any polymer generated in conventional silicon nitrideetch processes. The amount of any polymer, if present, in theconventional silicon nitride etch processes is typically not measurableby analytical instruments. In contrast, the amount of the firstfluorohydrocarbon-containing polymer is typically measurable byanalytical instruments such as an Auger electron spectrometer.

Because fluorohydrocarbon-containing polymers are generated insignificant quantities in the anisotropic etch process of the presentdisclosure, the energy of the fluorohydrocarbon-containing plasma can besignificantly lowered relative to the energy employed for conventionalsilicon nitride etch processes. As used herein, the quantity of thefluorohydrocarbon-containing polymers is “significant” if thefluorohydrocarbon-containing polymers are measurable by analyticalequipments known in the art. Thus, the high selectivity of the siliconnitride etch process relative to silicon and silicon nitride can beemployed to reduce the energy of the ions in thefluorohydrocarbon-containing plasma so that less plasma damage occurs onvarious physically exposed surfaces of the first exemplary structure.Reducing the energy of the ions in the fluorohydrocarbon-containingplasma decreases the total amount of polymers, i.e., the first andsecond fluorohydrocarbon-containing polymers that are formed during theanisotropic etch process employing the fluorohydrocarbon-containingplasma.

Ions in the fluorohydrocarbon-containing plasma can have any energyemployed in conventional plasma etching of silicon nitride, whichrequires minimum ion energy of 200 eV in order to etch silicon nitridein any significant manner. In contrast, the ions in thefluorohydrocarbon-containing plasma can have an energy less than 200 eV.Specifically, the ions in the fluorohydrocarbon-containing plasma of thepresent disclosure can have an average kinetic energy between 10 eV and1 keV. In one embodiment, the ions in the fluorohydrocarbon-containingplasma of the present disclosure can have an average kinetic energy in arange from 10 eV to 100 eV.

In one embodiment, the first fluorohydrocarbon-containing polymer layer30 is not etchable with any fluorohydrocarbon-containing plasma in theabsence of oxygen and at a plasma energy less than 1 keV. Once theanisotropic etch process has been completed and the structure shown inFIG. 2A has been obtained, the fluorohydrocaron-containing polymers arecleaned, for example, by a wet clean process. In one exemplaryembodiment, a remote hydrogen plasma is followed by buffered oxide etch.FIG. 2B schematically illustrates the resulting structure followingremoval of the polymer layers.

FIG. 3 schematically illustrates a starting structure similar to thatshown in FIG. 1 that includes a substrate 12, a gate stack having asilicon nitride cap 18, and source/drain regions 14 operativelyassociated with the gate stack. The structure further includes apatterned photoresist (PR) mask 24 having an opening 26 that exposes thegate stack and source/drain regions. The PR mask or trilayer stack usedis eroded during the conventional plasma due to O₂ admixture in theprocess and the typically higher amount of ion bombardment needed forselective etch. Using conventional anisotropic etch processing usingsingle carbon CH_(x)F_(y) plasma, the soft mask 24 is reduced inthickness, as shown schematically in FIG. 4A, and the dimensions of theopening 26 are enlarged. The enlarged opening 26′ exposes a largerportion of the top surface of the substrate. As discussed above, thesource/drain regions and gate stack may also be damaged by theanisotropic etch. In contrast, a structure as schematically illustratedin FIG. 4B is formed using the etch technology as described above withrespect to FIGS. 2A and 2B. Protective polymer layers are selectivelyformed on both the mask 24 and the source/drain regions as the siliconnitride cap etched. Selective polymer deposition on the PR mask preventserosion thereof (laterally and vertically). The deposit is sufficientlyresistant to withstand attack from an O₂ admixture. As shown in FIG. 4B,the mask thickness and the dimensions of the opening 26 remainsubstantially the same following the anisotropic etch that removes thesilicon nitride cap 18.

FIG. 5 is a flowchart showing steps that are performed in sequence inone exemplary embodiment of the method 50. A gate stack is formed on asemiconductor substrate. Semiconductor materials such as single crystalsilicon form the substrate in some exemplary embodiments. In anexemplary gate-first process, gate materials may comprise a gatedielectric (for example, high-k dielectric materials such as hafniumoxide or layers of dielectric materials) and a gate conductor (e.g.,metal gate). Any suitable deposition technique can be used to depositthe selected gate dielectric material(s) and electrically conductivegate materials, including but not limited to atomic layer deposition,chemical vapor deposition, physical vapor deposition, and sputtering.The deposited layers are lithographically patterned to form the stack,as known in the art. Dielectric spacers are formed around the gatestack. Such spacers can be formed by depositing a silicon nitride layeron the structure and then removing the horizontal portions of thedeposited silicon nitride layer using an anisotropic etch. Theanisotropic etch can be conducted using a single-carbonhydrofluorocarbon plasma or by forming a plasma from a gas comprisingC_(x)H_(y)F_(z), wherein x is an integer selected from 3, 4, 5, and 6, yand z are positive integers, and y is greater than z, as describedabove. Silicon nitride layers may be deposited using conventionaldeposition techniques such as spin-on coating, CVD, plasma-assisted CVD,or other known techniques. As indicated above, the spacers 16 in someembodiments may comprise materials other than silicon nitride ormultiple layers of different materials that exhibit dielectricproperties. A silicon nitride cap is formed on the gate stack prior toformation of the spacers. As shown in FIG. 1, the cap 18 may extend atleast partially above the spacers 16. The cap material is depositedbefore spacer formation, and as such is deposited uniformly onto thewafer and then patterned. The spacer material is deposited isotropicallyafter the gate stack and cap are formed. Once the silicon nitride caphas been formed on the gate stack, the source/drain regions are formed.The epitaxial deposition of doped semiconductor layers is employed inone or more exemplary embodiments as at least part of the process offorming source/drain regions having conductivity types opposite to theconductivity type of the channel region beneath the gate stack. It willbe appreciated that silicon nitride caps can be useful with respect tothe production of devices having source/drain regions formed using othertechniques such as diffusion and implantation as they facilitateseparate engineering of source/drain and gate regions. In thefabrication of a pFET structure, boron-doped SiGe can be formedepitaxially on a silicon substrate in some embodiments. The doping canbe chosen as desired for particular transistor applications. In oneexemplary embodiment where the doped source/drain semiconductor materialis SiGe, the dopant is boron in a concentration ranging 4-5e20 and theresulting FET structure is p-type. In an exemplary nFET structure, thesource/drain regions comprise carbon doped silicon having a carbondoping level of about 1.5%. The formation of source/drain regions usingone or more techniques is well known to those of skill in the art.Chemical vapor deposition may be employed for the epitaxial depositionof doped semiconductor layers. In a silicon-containing substrate,examples of p-type dopants, i.e., impurities include but are not limitedto: boron, aluminum, gallium and indium. As used herein, “n-type” refersto the addition of impurities that contributes free electrons to anintrinsic semiconductor. In a silicon containing substrate, examples ofn-type dopants, i.e., impurities, include but are not limited toantimony, arsenic and phosphorous. CMOS processing techniques familiarto those of skill in the art may be employed for the deposition ofsource and drain regions. An in situ doped epitaxial deposition processforms the source/drain regions 14 in some embodiments. A chemical vapordeposition (CVD) reactor may be used to cause the epitaxial growth ofchosen materials. Rapid thermal annealing may be employed to causediffusion of dopants into the substrate regions beneath the spacers. Ananisotropic plasma etch is employed once a structure such as shown inFIG. 1 is obtained. In some embodiments, a soft mask is deposited priorto the anisotropic etch. By forming a plasma from a gas comprisingC_(x)H_(y)F_(z) as described above that causes the removal of thesilicon nitride cap while selectively depositing protective polymerlayers on other portions of the substrate, including the source/drainregions, the cap is removed without causing unacceptable damage to thesource/drain regions. The gate stack is not damaged by this procedurebecause a protective polymer layer is selectively deposited on the gateconductor or another non-nitride layer of the gate stack once the caphas been etched away. In embodiments including a patterned photoresistlayer, the plasma etch as disclosed herein allows the dimensions of thephotoresist layer to remain substantially as formed. The structure shownin FIG. 2A is obtained in some embodiments following the selectiveanisotropic etch. The protective polymer layers are removed to obtain astructure as shown in FIG. 2B. Further processing (not shown), includingback-end-of-line (BEOL) processing follows completion of the steps shownin FIG. 5. It will be appreciated that the processing may conducted inwafer scale in some embodiments. Shallow trench isolation (STI) regions(not shown) may be formed in some embodiments by patterning techniquesfamiliar to those skilled in the art that facilitate trench formationand subsequent filling of the trenches with one or more electricallyinsulating material(s) such as silicon dioxide. Shallow trench isolation(STI) provides electrical isolation of active areas of the resultingstructure. The selective deposition of protective polymer layer(s) isapplicable the formation of various types of devices includingsource/drain regions and associated gate structures, including but notlimited to FinFET devices, JFETs, and bipolar devices.

Given the discussion thus far and with reference to the exemplaryembodiments discussed above and the drawings, it will be appreciatedthat, in general terms, an exemplary fabrication method includesobtaining a FET structure comprising a semiconductor substrate, a gatestack on the substrate, source/drain regions operatively associated withthe gate stack, and a silicon nitride cap of the gate stack. FIG. 1schematically illustrates such a structure. Afluorohydrocarbon-containing plasma selective to silicon bydecomposition of C_(x)H_(y)F_(z) wherein x is an integer selected from3, 4, 5 and 6, y and z are positive integers, and y is greater than z,is generated. The method further includes anisotropically etching thesilicon nitride cap employing the fluorohydrocarbon-containing plasma toform a first hydrofluorocarbon polymer layer having a first thickness onthe source/drain regions and a second hydrofluorocarbon polymer layerhaving a second thickness on the silicon nitride cap, the firstthickness being greater than the second thickness, the secondhydrofluorocarbon polymer layer further comprising a volatilenitrogen-containing compound formed by interaction of thefluorohydrocarbon-containing plasma with the silicon nitride comprisingthe silicon nitride cap. FIG. 2A shows the generation of the firsthydrofluorocarbon polymer layer 30 on the exemplary structure. One ormore embodiments of the exemplary method further include removing thefirst hydrofluorocarbon polymer layer from the source/drain regions.FIG. 2B shows an exemplary structure following such removal. One or morefurther embodiments of the method include forming a photoresist layerhaving a selected thickness on the FET structure and patterning thephotoresist to form an opening having selected dimensions and exposingthe silicon nitride cap and the source/drain regions. In embodimentsincluding formation of a photoresist layer, the method may furtherinclude including maintaining the selected thickness and the selecteddimensions of the photoresist layer during the step of anisotropicallyetching the silicon nitride cap, as shown schematically in FIG. 4B.

An exemplary structure provided in accordance with the disclosureincludes a semiconductor substrate, a gate stack on the substrate, achannel region beneath the gate stack, spacers adjoining the gate stack,and source/drain regions operatively associated with the gate stack andchannel region. A fluorohydrocarbon-containing polymer layer directlycontacts and covers the top surfaces of the source/drain regions. FIG.2A shows an exemplary structure having In one or more embodiments, thesemiconductor substrate is silicon-based. In one or more embodiments,the polymer layer comprises carbon at an atomic concentration betweenthirty and forty percent, hydrogen at an atomic concentration betweenforty and fifty percent, fluorine at an atomic concentration betweenfive and ten percent, and oxygen at an atomic concentration less thanfive percent. In some embodiments, the polymer layer has a thicknessbetween 0.1 to three nanometers. A patterned photoresist layer adjoinsthe substrate in some embodiments. The patterned photoresist layerincludes an opening in alignment with the gate stack and source/drainregions.

Those skilled in the art will appreciate that the exemplary structuresdiscussed above can be distributed in raw form or incorporated as partsof intermediate products or end products that benefit from having FETdevices therein.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, steps, operations, elements, and/orcomponents, but do not preclude the presence or addition of one or moreother features, steps, operations, elements, components, and/or groupsthereof. Terms such as “above” and “below” are used to indicate relativepositioning of elements or structures to each other as opposed torelative elevation.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the various embodiments has been presented for purposesof illustration and description, but is not intended to be exhaustive orlimited to the forms disclosed. Many modifications and variations willbe apparent to those of ordinary skill in the art without departing fromthe scope and spirit of the invention. The embodiments were chosen anddescribed in order to best explain the principles of the invention andthe practical application, and to enable others of ordinary skill in theart to understand the various embodiments with various modifications asare suited to the particular use contemplated.

What is claimed is:
 1. A structure comprising: a semiconductorsubstrate; a gate stack on the substrate; a channel region beneath thegate stack; spacers adjoining the gate stack; source/drain regionsoperatively associated with the gate stack and channel region, thesource/drain regions having top surfaces, and afluorohydrocarbon-containing polymer layer directly contacting andcovering the top surfaces of the source/drain regions.
 2. The structureof claim 1, wherein the semiconductor substrate is silicon-based.
 3. Thestructure of claim 1, wherein the fluorohydrocarbon-containing polymerlayer comprises carbon at an atomic concentration between thirty andforty percent, hydrogen at an atomic concentration between forty andfifty percent, fluorine at an atomic concentration between five and tenpercent, and oxygen at an atomic concentration less than five percent.4. The structure of claim 3, wherein the fluorohydrocarbon-containingpolymer layer has a thickness between 0.1 to three nanometers.
 5. Thestructure of claim 4, further including a patterned photoresist layeradjoining the substrate, the patterned photoresist layer including anopening in alignment with the gate stack and source/drain regions. 6.The structure of claim 1, wherein the source/drain regions comprise anepitaxial layer of a doped semiconductor material.
 7. The structure ofclaim 1, further including a second hydrofluorocarbon polymer layercomprising a volatile nitrogen-containing compound on the gate stack.